1. Field of the Invention
This invention relates in general to voltage controlled oscillators (VCO's), and more particularly to reduction of phase noise in the frequency spectrum around a VCO signal frequency.
2. Description of the Prior Art
As is well known in the art, a dual band VCO selects between oscillator output frequencies by switching resonant circuit elements in the active circuit. For each output frequency selected, the oscillator produces a single frequency signal and additional energy in the form of phase noise. This additional sideband noise produced by modulation of the single frequency, is produced by the active device in the oscillator as flicker noise, or the noise figure of the active device under large signal conditions and the filtering effect or the resonant circuit. Achievement of good phase noise performance in each of the switched bands, by use of separate VCO's, has the disadvantage of requiring more components, weight, and cost. Phase noise, as it is known and understood by those skilled in the art, is discussed in 7.5 Noise and Performance Analyses Using CAD Tools, Communications Receivers. 2nd Edition, Ulrich L. Rohde, Jerry Whitaker and T. T. N. Bucher, which is published by McGraw Hill Company. However, changing the resonant elements without more, does not achieve optimum phase noise performance in a dual band VCO.
Therefore, there exists a need to resolve the problems with the prior art to significantly improve phase noise performance of a dual band VCO as discussed above.